Semi-Permeable Membranes- An "Egg"cellent
Model (No Yolk!)

Yes, biologists are interested in particles,
too!

Background: In all living organisms, vital fluids and
particles of solids too small to see must be able to cross membranes.
These same membranes must be capable of excluding other materials.
As organisms metabolize food, grow, and perform all the functions
of life, semi-permeable membranes provide access to and protection
from countless particles of nature. Many students (and some math
teachers) have considerable difficulty conceptualizing the process
of particles too small to see being allowed or obstructed passage
through the walls of capillaries, intestinal villi, cell membranes,
and other "biological fences." This model provides a
concrete example.

Grade Level: This activity is adapted from Science Plus,
an STS middle school curriculum developed by Holt, Reinhart, and
Winston. It could easily be modified for any grade level.

Materials: For each lab partnership:

1 - 100 ML or 250 ML beaker

5 ML of cornstarch

5 ML of dextrose (do not use table sugar)

Iodine solution

Benedict's solution

Hot plate & 1.5 L kettle (1 set per classroom)

Graduated cylinder (10ML)

Large test tube and stopper (to mix sugar & starch)

2 small test tubes and test tube holder

Eye droppers (to place water in floating shell, add iodine,
add Benedict's) (DROPERS MUST BE KEPT SEPARATE BY TASK!!)

Egg and straight pin (I found a tiny screw driver used for
eye glasses worked well to create the hole in the egg shell)

Stirring rod

Procedure: 1. Shake 5 ML cornstarch and 5 ML dextrose
in test tube to mix. Add to 100 ML HOT water. Stir well
2. Crack egg in half, saving large end containing air sac. (I
did this for the students while they set up their experiment.
I salvage the "innards' in a clean container and made lots
of souffles!") Do not break eggs in advance as membranes
may dehydrate and crack. Trust me!
3. Using a straight pin, CAREFULLY pierce the large end of the
egg shell. Do not break the top end of the air sac membrane. (Yes,
the kids really can do this! If the pin does not work, have the
tiny screw driver available. Prior sanding the end of the egg
also helped.)
4. Using clean eye dropper, place 5 - 10 ML of water into the
egg shell. Float the shell "boat" in the sugar/cornstarch
mixture.
5. After 15 minutes, use clean eye dropper to divide water in
the shell and place equally into the small test tube.
6. Test one half of the liquid with iodine. (A blue/purple color
indicates starch.)
7. Add 8 drops of Benedict's solution to the other half of the
"egg water." CAUTION: Benedict's solution is a corrosive. Goggles should
be in place! Heat the liquid in a hot water bath - GENTLY! A yellow/orange/red
color indicates a positive reaction for sugar. (This may take
a few minutes.)

I demonstrated positive reactions for the class using mixture.

Teacher Note: Due to time constraints, I had students complete
steps 1-5 on one day and stopper the tubes. We tested and discussed
on the next day. The only problem was in labeling and storing
100+ test tubes overnight.

Discussion Questions:

1. In which test tube did a positive reaction occur? (Sugar)

2. What property may have contributed to a positive reaction with
the sugar but not the starch? (sugar molecules may be smaller
and, therefore, able to pass through the membrane. Starch molecules
may be too large to pass through. Different molecules, though
microscopic, may differ greatly in size.)

3. What may be the purpose of the air sac membrane? (Let air in
for the embryo to utilize. Allow exchange of oxygen and carbon
dioxide. Keep moisture in the egg so the embryo will not dehydrate.)

4. Name some of the other membranes that organisms, including
man, use to allow some substances in while keeping other materials
out. (Plant cell walls, vacuoles, animal cell membranes, intestinal
villi, capillaries, etc.)

5. Man metabolizes starch and sugar. How can we absorb starch
into our systems if the molecules are too large to pass across
membranes? (Springboard for discussion into digestion! Watch for
nextmonth's activity!)

6. If you tested for starch and found a positive reaction, what
is you justification? (Pierced shell membrane provided a "door"
for the larger particles.)

7. How would you justify no reaction to sugar? (Human error. Failure
to follow directions, not heatingsolution long enough.)